First, is CS6 updated to version 8.0.2? Will the IDML file open in CC?
That particular error generally indicates that the file is either not an InDesign file at all, there is a lock file in the folder (not an issue for IDML), or it is corrupt.
If you are a CC subscriber, install CS6 from the cloud (the permanent version will not help here) and open the CC or CC2014 file in CS6 as it will connect to Adobe's server and convert it to CS6 INDD file. This INDD can be saved and now be opened in any CS6, permanent and cloud version.